High numerical aperture multimode optical fiber

ABSTRACT

Bend resistant multimode optical fibers are disclosed herein. Multimode optical fibers disclosed herein comprise a core region having a radius greater than 30 microns and a cladding region surrounding and directly adjacent to the core region, the cladding region comprising a depressed-index annular portion comprising a depressed relative refractive index. The fiber has a total outer diameter of less than 120 microns, and exhibits an overfilled bandwidth at 850 nm greater than 500 MHz-km.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of, and priority to U.S. ProvisionalPatent Application No. 61/100,404 filed on Sep. 26, 2008 and U.S.Provisional Patent Application No. 61/154,579 filed on Feb. 23, 2009 andU.S. Provisional Patent Application No. 61/224,676 filed on Jul. 10,2009 entitled, “High Numerical Aperture Multimode Optical Fiber”, thecontent of which is relied upon and incorporated herein by reference inits entirety.

FIELD OF THE INVENTION

The present invention relates generally to optical fibers, and morespecifically to multimode optical fibers.

TECHNICAL BACKGROUND

Corning Incorporated manufactures and sells InfiniCor® 62.5 μm opticalfiber, which is multimode optical fiber having a core with a maximumrelative refractive index delta of about 2% and 62.5 μm core diameter,as well as InfiniCor® 50 μm optical fiber, which is multimode opticalfiber having a core with a maximum relative refractive index delta ofabout 1% and 50 μm core diameter. Corning also manufactured 100/140 CPC3Multimode Fiber, which is a graded index optical fiber with a 100 μmcore diameter, a 140 μm undoped silica cladding diameter and a numericalaperture of 0.29.

SUMMARY OF THE INVENTION

Bend resistant multimode optical fibers are disclosed herein. Multimodeoptical fibers disclosed herein comprise a graded-index core regionhaving a core radius greater than 30 microns and a cladding regionsurrounding the core region, the cladding region comprising adepressed-index annular cladding region which is depressed relative toanother portion of the cladding. The core of the fiber is preferablyglass, and the depressed index annular region and any cladding regionsmay also be glass. The depressed-index annular cladding regionpreferably has a refractive index delta less than about −0.1 and a widthof at least 1 micron, more preferably a refractive index delta less thanabout −0.2 and a width of at least 2 microns. The depressed-indexannular cladding region is preferably directly adjacent to the core.However, the depressed-index annular cladding region may be spaced fromthe core, for example by an amount less than 4 microns, more preferablyby 1 to 4 microns. Such optical fibers which are disclosed herein arecapable of exhibiting an overfilled bandwidth at 850 nm which is greaterthan 500 MHz-km. The fibers disclosed herein preferably comprise anoutermost glass diameter of less than 120 microns, more preferably lessthan 110 microns.

Preferably, the refractive index profile of the core has a parabolic orsubstantially parabolic shape. The depressed-index annular portion may,for example, comprise glass comprising a plurality of voids, or glassdoped with a downdopant such as fluorine, boron or mixtures thereof, orglass doped with one or more of such downdopants and additionally glasscomprising a plurality of voids. In some preferred embodiments, thedepressed-index annular portion is comprised of fluorine doped silicaglass. In some embodiments, the depressed-index annular portion has arefractive index delta less than about −0.2% and a width of at least 1micron, more preferably less than about −0.24% and a width of at least 2microns.

In some embodiments that comprise a cladding with voids, the voids insome preferred embodiments are non-periodically located within thedepressed-index annular portion. By “non-periodically located”, we meanthat when one takes a cross section (such as a cross sectionperpendicular to the longitudinal axis) of the optical fiber, thenon-periodically disposed voids are randomly or non-periodicallydistributed across a portion of the fiber (e.g. within thedepressed-index annular region). Similar cross sections taken atdifferent points along the length of the fiber will reveal differentrandomly distributed cross-sectional hole patterns, i.e., various crosssections will have different hole patterns, wherein the distributions ofvoids and sizes of voids do not exactly match. That is, the voids orvoids are non-periodic, i.e., they are not periodically disposed withinthe fiber structure. These voids are stretched (elongated) along thelength (i.e. parallel to the longitudinal axis) of the optical fiber,but do not extend the entire length of the entire fiber for typicallengths of transmission fiber. It is believed that the voids extendalong the length of the fiber a distance less than 20 meters, morepreferably less than 10 meters, even more preferably less than 5 meters,and in some embodiments less than 1 meter.

The multimode optical fiber disclosed herein exhibits very low bendinduced attenuation, in particular very low macrobending inducedattenuation. Consequently, the multimode optical fiber may comprise agraded index glass core; and a second cladding comprising adepressed-index annular portion surrounding the core, saiddepressed-index annular portion having a refractive index delta lessthan about −0.2% and a width of at least 1 micron, more preferably lessthan about −0.24% and a width of at least 2 microns, wherein the fiberfurther exhibits a 1 turn 10 mm diameter mandrel wrap attenuationincrease, of less than or equal to 0.4 dB/turn at 850 nm, a numericalaperture of greater than 0.18, more preferably greater than 0.2, andmost preferably greater than 0.24. and an overfilled bandwidth greaterthan 700 MHz-km at 850 nm, more preferably greater than 1000 MHz-km at850 nm, more preferably greater than1500 MHz-km at 850 nm.

Using the designs disclosed herein, 60 micron or greater (e.g., greaterthan 70 microns, or greater than 75 microns) diameter core multimodefibers can been made which provide (a) an overfilled (OFL) bandwidth ofgreater than 700 MHz-km at 850 nm, more preferably greater than 1000MHz-km at 850 nm, more preferably greater than 1500 MHz-km at 850 nm ata wavelength of 850 nm. These high bandwidths can be achieved whilestill maintaining a 1 turn 10 mm diameter mandrel wrap attenuationincrease at a wavelength of 850 nm, of less than 1.5 dB, more preferablyless than 1.0 dB, even more preferably less than 0.8 dB, and mostpreferably less than 0.5 dB. These high bandwidths can also be achievedwhile also maintaining a 1 turn 20 mm diameter mandrel wrap attenuationincrease at a wavelength of 850 nm, of less than 1.0 dB, more preferablyless than 0.7 dB, and most preferably less than 0.5 dB, and a 1 turn 15mm diameter mandrel wrap attenuation increase at a wavelength of 850 nm,of less than 1.2 dB, preferably less than 1.0 dB, and more preferablyless than 0.8 dB. Such fibers are further capable of providing anumerical aperture (NA) greater than 0.18, more preferably greater than0.2, and most preferably greater than 0.24. Such fibers are furthersimultaneously capable of exhibiting an OFL bandwidth at 1300 nm whichis greater than 200 MHz-km, preferably greater than 500 MHz-km, morepreferably greater than 1000 MHz-km and most preferably greater than1500 MHz-km.

Using the designs disclosed herein, 60 micron or greater (e.g., greaterthan 70 microns, more preferably greater than 75 microns) diameter coremultimode fibers can be made which provide (a) an overfilled (OFL)bandwidth of greater than 500 MHz-km at 850 nm, more preferably greaterthan 700 MHz-km at 850 nm, more preferably greater than 1000 MHz-km at850 nm at a wavelength of 850 nm. These high bandwidths can be achievedwhile still maintaining a 1×180° turn 3 mm diameter mandrel wrapattenuation increase at a wavelength of 850 nm, of less than 1.0 dB,more preferably less than 0.5 dB, even more preferably less than 0.3 dB.These high bandwidths can also be achieved while still maintaining a2×90° turn 4 mm diameter mandrel wrap attenuation increase at awavelength of 850 nm, of less than 0.5 dB, more preferably less than 0.2dB, even more preferably less than 0.1 dB. These bend losses andbandwidths are achieved both when the input signal is aligned with thecenter of the fiber, as well as when the input signal is launched withan offset of 5 or even 10 μm relative to the center of the fiber.

Preferably, the multimode optical fiber disclosed herein exhibits aspectral attenuation of less than 3.5 dB/km at 850 nm, preferably lessthan 3.0 dB/km at 850 nm, even more preferably less than 2.7 dB/km at850 nm and still more preferably less than 2.5 dB/km at 850 nm.Preferably, the multimode optical fibers disclosed herein exhibit aspectral attenuation of less than 1.5 dB/km at 1300 nm, preferably lessthan 1.2 dB/km at 1300 nm, even more preferably less than 0.8 dB/km at1300 nm. It may be desirable to spin the multimode fiber, as doing somay in some circumstances further improve the bandwidth for opticalfiber having a depressed cladding region. By spinning, we mean applyingor imparting a spin to the fiber wherein the spin is imparted while thefiber is being drawn from an optical fiber preform, i.e. while the fiberis still at least somewhat heated and is capable of undergoingnon-elastic rotational displacement and is capable of substantiallyretaining the rotational displacement after the fiber has fully cooled.

The numerical aperture (NA) of the optical fibers disclosed herein arepreferably less than 0.32 and greater than 0.18, more preferably greaterthan 0.2, even more preferably less than 0.32 and greater than 0.24, andmost preferably less than 0.30 and greater than 0.24.

The core may be designed to extend radially outwardly from thecenterline to a radius R1, R1≧30 microns, more preferably R1≧40 microns,and in some cases R1≧45 microns. The core may be designed to have R1≦50microns, and preferably R1≦45 microns.

The core may be designed to have a maximum relative refractive index,less than or equal to 2.5% and greater than 0.5%, more preferably lessthan 2.2% and greater than 0.9%, most preferably less than 1.8% andgreater than 1.2%. The core may have a maximum refractive index between1.6 and 2.0%.

Optical fibers disclosed herein are capable of exhibiting a 1 turn 10 mmdiameter mandrel attenuation increase of no more than 1.5 dB, preferablyno more than 1.0 dB, more preferably no more than 0.8 dB, and mostpreferably no more than 0.5 dB, at all wavelengths between 800 and 1400nm.

The multimode optical fibers disclosed herein may comprise agraded-index glass core, disposed about a longitudinal centerline, and aglass cladding surrounding the core. The cladding comprises adepressed-index annular portion, and an outer annular portion. Thedepressed-index annular portion preferably directly abuts the core, andin some embodiments, the outer annular portion preferably comprisesundoped silica cladding, although in other embodiments the depressedannular portion may extend to the outermost glass diameter of theoptical fiber. All refractive indices set forth herein are in referenceto the outer annular portion as described below.

The fibers disclosed herein are capable of being multimoded at theconventional operating wavelengths for such telecommunications fibers,i.e., at least over the wavelength range extending from 850 nm to 1300nm.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of the invention,and are intended to provide an overview or framework for understandingthe nature and character of the invention as it is claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention, and are incorporated into and constitute a part of thisspecification. The drawings illustrate various embodiments of theinvention, and together with the description serve to explain theprinciples and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation (not to scale) of the refractiveindex profile of a cross-section of the glass portion of an exemplaryembodiment of multimode optical fiber disclosed herein wherein thedepressed-index annular portion is offset from the core and issurrounded by an outer annular cladding portion.

FIG. 2 is a schematic representation (not to scale) of a cross-sectionalview of the optical waveguide fiber of FIG. 1.

FIG. 3 shows a schematic representation (not to scale) of the refractiveindex profile of a cross-section of the glass portion of an exemplaryembodiment of multimode optical fiber disclosed herein wherein thedepressed-index annular portion is not offset from the core and issurrounded by an outer annular cladding portion.

FIG. 4 is a schematic representation (not to scale) of a cross-sectionalview of the optical waveguide fiber of FIG. 3.

DETAILED DESCRIPTION

Additional features and advantages of the invention will be set forth inthe detailed description which follows and will be apparent to thoseskilled in the art from the description or recognized by practicing theinvention as described in the following description together with theclaims and appended drawings.

The “refractive index profile” is the relationship between refractiveindex or relative refractive index and waveguide fiber radius.

The “relative refractive index percent” is defined as Δ%=100×(n_(i)²−n_(REF) ²)/2n_(i) ², where n_(i) is the maximum refractive index inregion i, unless otherwise specified. The relative refractive indexpercent is measured at 850 nm unless otherwise specified. Unlessotherwise specified herein, n_(REF) is the refractive index of undopedsilica glass, i.e. 1.4533 at 850 nm.

As used herein, the relative refractive index is represented by Δ andits values are given in units of “%”, unless otherwise specified. Incases where the refractive index of a region is less than the referenceindex n_(REF), the relative index percent is negative and is referred toas having a depressed region or depressed-index, and the minimumrelative refractive index is calculated at the point at which therelative index is most negative unless otherwise specified. In caseswhere the refractive index of a region is greater than the referenceindex n_(REF), the relative index percent is positive and the region canbe said to be raised or to have a positive index. An “updopant” isherein considered to be a dopant which has a propensity to raise therefractive index relative to pure undoped SiO₂. A “downdopant” is hereinconsidered to be a dopant which has a propensity to lower the refractiveindex relative to pure undoped SiO₂. An updopant may be present in aregion of an optical fiber having a negative relative refractive indexwhen accompanied by one or more other dopants which are not updopants.Likewise, one or more other dopants which are not updopants may bepresent in a region of an optical fiber having a positive relativerefractive index. A downdopant may be present in a region of an opticalfiber having a positive relative refractive index when accompanied byone or more other dopants which are not downdopants. Likewise, one ormore other dopants which are not downdopants may be present in a regionof an optical fiber having a negative relative refractive index.

Unless otherwise noted, macrobend performance was determined accordingto FOTP-62 (IEC-60793-1-47) by wrapping 1 turn around either a 6 mm, 10mm, or 20 mm or similar diameter mandrel (e.g. “1×10 mm diametermacrobend loss” or the “1×20 mm diameter macrobend loss”) and measuringthe increase in attenuation due to the bending using an overfilledlaunch condition where the optical source has a spot size that isgreater than 50% of the core diameter of the fiber under test. In somemeasurements, an encircled flux launch (EFL) macrobend performance wasobtained by launching an overfilled pulse into an input end of a 2 mlength of InfiniCor® 50 μm optical fiber which was deployed with a 1×25mm diameter mandrel near the midpoint. The output end of the InfiniCor®50 μm optical fiber was spliced to the fiber under test, and themeasured bend loss is the ratio of the attenuation under the prescribedbend condition to the attenuation without the bend.

As used herein, numerical aperture of the fiber means numerical apertureas measured using the method set forth in TIA SP3-2839-URV2 FOTP-177IEC-60793-1-43 titled “Measurement Methods and Test Procedures-NumericalAperture”.

The term “a-profile” or “alpha profile” refers to a relative refractiveindex profile, expressed in terms of Δ(r) which is in units of “%”,where r is radius, which follows the equation,Δ(r)=Δ(r _(o))(1−[|r−r _(o)|/(r ₁ −r _(o))]^(α)),where r_(o), is zero unless otherwise specified, r₁ is the point atwhich Δ(r) % is zero, and r is in the range r_(i)≦r≦r_(f), where Δ isdefined above, r_(i) is the initial point of the α-profile, r_(f) is thefinal point of the α-profile, and α is an exponent which is a realnumber.

The depressed-index annular portion has a profile volume, V₃, definedherein as:

2∫_(R_(INNER))^(R_(OUTER))Δ₃(r)r𝕕rwhere R_(INNER) is the depressed-index annular portion inner radius andR_(OUTER) is the depressed-index annular portion outer radius asdefined. For the fibers disclosed herein, the absolute magnitude of V₃is preferably greater than 120%-μm², more preferably greater than160%-μm², and even more preferably greater than 200%-μm². Preferably theabsolute magnitude of V₃ is less than 400%-μm², more preferably lessthan 300%-μm². In some preferred embodiments, the absolute magnitude ofV₃ is greater than 120%-μm² and less than 300%-μm². In other preferredembodiments, the absolute magnitude of V₃ is greater than 160%-μm² andless than 240%-μm².

Multimode optical fiber disclosed herein comprises a core and a claddingsurrounding and directly adjacent the core. In some embodiments, thecore comprises silica doped with germanium, i.e. germania doped silica.Dopants other than germanium such as Al₂O₃ or P₂O₅, singly or incombination, may be employed within the core, and particularly at ornear the centerline, of the optical fiber disclosed herein to obtain thedesired refractive index and density. In some embodiments, therefractive index profile of the optical fiber disclosed herein isnon-negative from the centerline to the outer radius of the core. Insome embodiments, the optical fiber contains no index-decreasing dopantsin the core.

FIG. 1 illustrates a schematic representation of the refractive indexprofile of a cross-section of the glass portion of one exemplaryembodiment of a multimode optical fiber comprising a glass core 20 and aglass cladding 200, the cladding comprising an inner annular portion 30,a depressed-index annular portion 50, and an outer annular portion 60.FIG. 2 is a schematic representation (not to scale) of a cross-sectionalview of the optical waveguide fiber of FIG. 1. The core 20 has outerradius R₁ and maximum refractive index delta Δ1_(MAX). The inner annularportion 30 is comprised of refractive index delta 42 and has width W₂and outer radius R₂. Depressed-index annular portion 50 has minimumrefractive index delta percent Δ3_(MIN), width W₃ and outer radius R₃.The depressed-index annular portion 50 is shown offset, or spaced away,from the core 20 by the inner annular portion 30. In preferredembodiments, the width of inner annular portion 30 may be less than 4.0microns.

In the embodiment illustrated in FIG. 1, the annular portion 50surrounds inner annular portion 30, and the outer annular claddingportion 60 surrounds and contacts annular portion 50. The inner annularportion 30 has a refractive index profile Δ2(r) with a maximum relativerefractive index Δ2_(MAX), and a minimum relative refractive indexΔ2_(MIN), where in some embodiments Δ2_(MAX)=Δ2_(MIN). Thedepressed-index annular portion 50 has a refractive index profile Δ3(r)with a minimum relative refractive index Δ3_(MIN). The outer annularportion 60 comprises relative refractive index Δ4. Preferably, Δ1>Δ4>Δ3,and in the embodiment illustrated in FIG. 1, Δ1>Δ2>Δ3. In someembodiments, the inner annular portion 30 has a substantially constantrefractive index profile, as shown in FIG. 1 with a constant Δ2(r); insome of these embodiments, Δ2(r)=0%. In some embodiments, the outerannular portion 60 has a substantially constant refractive indexprofile, as shown in FIG. 1 with a constant Δ4(r); in some of theseembodiments, Δ4(r)=0%. Preferably, the core contains substantially nofluorine, and preferably the core contains no fluorine. In someembodiments, the inner annular portion 30 preferably has a relativerefractive index profile Δ2(r) having a maximum absolute magnitude lessthan 0.05%, and Δ2_(MAX)<0.05% and Δ2_(MIN)>−0.05%, and thedepressed-index annular portion 50 begins where the relative refractiveindex of the cladding first reaches a value of less than −0.05%, goingradially outwardly from the centerline. In some embodiments, the outerannular portion 60 has a relative refractive index profile Δ4(r) havinga maximum absolute magnitude less than 0.05% and greater than −0.05%.

The outer diameter of the glass portion of the optical fiber ispreferably less than 120 μm, more preferably less than 110 μm, even morepreferably less than or equal to about 100 μm. Thus, in the embodimentillustrated in FIG. 1, the outer cladding diameter (2×R₄) is preferablyless than 120 μm, more preferably less than 110 μm, even more preferablyless than 100 μm. In some embodiments, the core diameter (2×R₁) isbetween 35 and 45 μm, more preferably 37 and 43 μm, and the outercladding diameter R₄ is between 45 and 55 μm, more preferably between 47and 53 μm. In some preferred embodiments, the outer cladding region 60has a width less than 15 μm, more preferably less than 10 μm, mostpreferably less than 7 μm.

In the multimode optical fiber disclosed herein, the core is agraded-index core, and preferably, the refractive index profile of thecore has a parabolic (or substantially parabolic) shape; for example,the refractive index profile of the core may have an α-shape with an αvalue preferably between 1.9 and 2.3, more preferably about 2.1, asmeasured at 850 nm Alternatively, the refractive index profile of thecore may have an α-shape with an α value preferably between 1.9 and 2.1,more preferably about 2.0, as measured at 850 nm. In some embodiments,the refractive index of the core may have a centerline dip, wherein themaximum refractive index of the core, and the maximum refractive indexof the entire optical fiber, is located a small distance away from thecenterline, but in other embodiments the refractive index of the corehas no centerline dip, and the maximum refractive index of the core, andthe maximum refractive index of the entire optical fiber, is located atthe centerline. The parabolic shape extends to a radius R₁ andpreferably extends from the centerline of the fiber to R₁. As usedherein, “parabolic” therefore includes substantially parabolicallyshaped refractive index profiles which may vary slightly from an α valueof about 2.0, for example 1.9, 2.1 or 2.3, at one or more points in thecore, as well as profiles with minor variations and/or a centerline dip.Referring to the Figures, the core 20 is defined to end at the radius R₁where the parabolic core shape ends, coinciding with the innermostradius of the cladding 200.

Preferably, the optical fiber disclosed herein has a silica-based glasscore and cladding. One or more portions of the clad layer 200 may becomprised of a cladding material which was deposited, for example duringa laydown process, or which was provided in the form of a jacketing,such as a tube in a rod-in-tube optical preform arrangement, or acombination of deposited material and a jacket. The clad layer 200 maybe surrounded by at least one coating 210, which may in some embodimentscomprise a low modulus primary coating and a high modulus secondarycoating. The coating can be a polymer coating such as an acrylate-basedpolymer.

In some embodiments, the depressed-index annular portion comprisesvoids, either non-periodically disposed, or periodically disposed, orboth. By “non-periodically disposed” or “non-periodic distribution”, wemean that when one takes a cross section (such as a cross sectionperpendicular to the longitudinal axis) of the optical fiber, thenon-periodically disposed voids are randomly or non-periodicallydistributed across a portion of the fiber. Similar cross sections takenat different points along the length of the fiber will reveal differentcross-sectional hole patterns, i.e., various cross sections will havedifferent hole patterns, wherein the distributions of voids and sizes ofvoids do not match. That is, the voids are non-periodic, i.e., they arenot periodically disposed within the fiber structure. These voids arestretched (elongated) along the length (i.e. parallel to thelongitudinal axis) of the optical fiber, but do not extend the entirelength of the entire fiber for typical lengths of transmission fiber.While not wishing to be bound by theory, it is believed that the voidsextend less than a few meters, and in many cases less than 1 meter alongthe length of the fiber. Optical fiber disclosed herein can be made bymethods which utilize preform consolidation conditions which areeffective to result in a significant amount of gases being trapped inthe consolidated glass blank, thereby causing the formation of voids inthe consolidated glass optical fiber preform. Rather than taking stepsto remove these voids, the resultant preform is used to form an opticalfiber with voids therein. As used herein, the diameter of a hole is thelongest line segment whose endpoints are disposed on the silica internalsurface defining the hole when the optical fiber is viewed inperpendicular cross-section transverse to the longitudinal axis of thefiber.

In some embodiments, the inner annular portion 30 comprises silica whichis substantially undoped with either fluorine or germania. Preferably,the annular portion 30 comprises a width of less than 4.0 microns, morepreferably less than 2.0 microns. In some embodiments, the outer annularportion 60 comprises substantially undoped silica, although the silicamay contain some amount of chlorine, fluorine, germania, or otherdopants in concentrations that collectively do not significantly modifythe refractive index. The depressed-index annular portion 50 maycomprise silica doped with fluorine and/or boron. Alternatively, thedepressed-index annular portion 50 may comprise silica comprising aplurality of non-periodically disposed voids. The voids can contain oneor more gases, such as argon, nitrogen, krypton, CO₂, SO₂, or oxygen, orthe voids can contain a vacuum with substantially no gas; regardless ofthe presence or absence of any gas, the refractive index in the annularportion 50 is lowered due to the presence of the voids. The voids can berandomly or non-periodically disposed in the annular portion 50 of thecladding 200, and in other embodiments, the voids are disposedperiodically in the annular portion 50. Alternatively, or in addition,the depressed index in annular portion 50 can also be provided bydowndoping the annular portion 50 (such as with fluorine) or updopingone or more portions of the cladding and/or the core, wherein thedepressed-index annular portion 50 is, for example, silica which is notdoped as heavily as the inner annular portion 30. Preferably, theminimum relative refractive index, or average effective relativerefractive index, such as taking into account the presence of any voids,of the depressed-index annular portion 50 is preferably less than −0.1%,more preferably less than about −0.2 percent, even more preferably lessthan about −0.3 percent, and most preferably less than about −0.4percent.

FIG. 3 illustrates a schematic representation of the refractive indexprofile of a cross-section of the glass portion of an alternativeexemplary embodiment of a multimode optical fiber comprising a glasscore 20 and a glass cladding 200, the cladding comprising adepressed-index annular portion 50, and an outer annular portion 60.FIG. 4 is a schematic representation (not to scale) of a cross-sectionalview of the optical waveguide fiber of FIG. 3. The core 20 has outerradius R₁ and maximum refractive index delta Δ1_(MAX). Thedepressed-index annular portion 50 has minimum refractive index deltapercent Δ3_(MIN), width W₃ and outer radius R₃. The depressed-indexannular portion 50 surrounds and is in direct contact with the core 20,i.e., there is no inner cladding region 30 (having Δ2) between the core20 and the depressed-index annular portion 50. Preferably, Δ1>Δ4>Δ3. Theouter annular portion 60 surrounds and contacts depressed-index annularportion 50. The depressed-index annular portion 50 has a refractiveindex profile Δ3(r) with a minimum relative refractive index Δ3_(MIN).The outer annular portion 60 has a refractive index profile Δ4(r) with amaximum relative refractive index Δ4_(MAX), and a minimum relativerefractive index Δ4_(MIN), where in some embodiments Δ4_(MAX)=Δ4_(MIN).Preferably, Δ1_(MAX)>Δ3_(MIN). Preferably, the core is doped withgermania and contains substantially no fluorine, more preferably thecore contains no fluorine. The depressed-index annular portion 50 beginswhere the relative refractive index of the cladding first reaches avalue of −0.05%, going radially outwardly from the centerline. In someembodiments, the outer annular portion 60 has a relative refractiveindex profile Δ4(r) having a maximum absolute magnitude less than 0.05%,and Δ4_(MAX)<0.05% and Δ4_(MIN)>−0.05%, and the depressed-index annularportion 50 ends where the region of relatively constant refractive index(Δ4) begins.

The numerical aperture (NA) of the optical fiber is preferably greaterthan the NA of the optical source directing signals into the fiber; forexample, the NA of the optical fiber is preferably greater than the NAof a VCSEL source.

If desired, a hermetic coating of carbon can be applied to the outerglass surface of the fibers disclosed herein. The coating may have athickness less than about 100 angstroms. At least one, and preferablytwo (a soft primary and a harder secondary coating) protective polymercoatings may be applied over the carbon coating. The use of such acarbon coating results in improved dynamic fatigue resistance, e.g.,using such coatings, dynamic fatigue constants greater than 50, morepreferably greater than 100 are achievable, and 15% Weibull failureprobabilities greater than 400 kpsi are achievable.

Set forth below in Table 1 are refractive index parameters and modeledoptical properties of a variety of examples. Examples 1-6 exhibitrefractive index profiles similar to those illustrated by FIG. 3.Examples 7 and 8 exhibit refractive index profiles similar to thoseillustrated by FIG. 1. In particular, provided below are delta 1 of coreregion 20, outer radius R1 of core region 20, alpha of core region 20,42 of inner annular region 30, outer radius R2 and width W2 of innerannular region 30, 43 of depressed index cladding region 50, outerradius R3 of depressed index cladding region 50, profile volume V3 ofdepressed index cladding region 50. Clad radius is the outermost radiusof the fiber (R₄) as well as the outer radius of the outer annular glasscladding portion 60. Also provided is the numerical aperture NA of thefiber and the product of the numerical aperture and the core diameter(NA*CD). NA*CD may preferably be greater than 20 μm, and more preferablygreater than 22 μm. In some embodiments NA*CD is less than 30 μm. Theouter glass cladding diameter is in some embodiments less than 120microns, preferably less than 110 microns.

TABLE 1 1 2 3 4 5 6 7 8 Delta1 (%) 0.95 0.95 1.1 1.3 1.4 2 1.4 1.8 R1(μm) 50.0 60.0 50.0 50.0 50.0 50.0 50.0 50.0 Alpha 2.1 2.1 2.1 2.1 2.12.1 2.1 2.1 Delta2 (%) 0 0 R2 (μm) 51.0 51.6 W2 (μm) 1.0 1.6 Delta3 (%)−0.39 −0.41 −0.54 −0.29 −0.39 −0.45 −0.35 −0.42 R3 (μm) 53.0 63.6 52.554.6 54.0 52.6 55.3 56.0 V3 (%-sq. μm) −120.5 −182.4 −138.4 −139.5−162.2 −120.0 −160.0 −200.0 Clad Radius 62.5 70.0 62.5 62.5 62.5 62.562.5 62.5 NA 0.2023 0.2023 0.218 0.2374 0.2467 0.2967 0.2467 0.2808NA*CD (μm) 20.2 24.3 21.8 23.7 24.7 29.3 24.3 28.1

Actual optical fiber Examples 9 through 14 are described as follows. Therefractive index profile parameters and properties of the resultantmultimode optical fibers made in accordance with these examples arefurther described in Table 2 below. In each case, conventional primaryand secondary urethane acrylate based protective coatings were appliedto the outside of the glass optical fibers.

Example 9 Comparative

650 grams of SiO₂ (0.36 g/cc density) soot were flame deposited onto a 1meter long×26.0 mm diameter solid glass cane of GeO₂—SiO₂ graded indexcore (0.96% maximum refractive index relative to pure silica with anearly parabolic (α=2.11) shape). This assembly was then sintered asfollows. The assembly was first dried for 2 hours in an atmosphereconsisting of helium and 3 percent chlorine at 1000° C. followed by downdriving at 6 mm/min through a hot zone set at 1500° C. in a 100 percenthelium atmosphere, in order to sinter the soot to an optical preformcomprising a GeO₂—SiO₂ graded index core and a silica outer cladding.The preform was placed for 24 hours in an argon purged holding oven setat 1000° C. The preform was drawn to a 10 km length of 125 microndiameter fiber at 10 m/s using a draw furnace having a hot zone of about8 cm length and set at approximately 2000° C. The measured OFLbandwidths of this fiber were 2315 and 666 MHz-km at 850 and 1300 nm,respectively. The macrobend loss at 850 nm measured on a 20 mm diametermandrel was 1.04 dB/turn, and the NA of the fiber was 0.193.

Example 10

673 grams of SiO₂ (0.36 g/cc density) soot were flame deposited onto a 1meter long×26.0 mm diameter solid glass cane of GeO₂—SiO₂ graded indexcore (1.02% maximum refractive index relative to pure silica) with anearly parabolic (α=2.16) shape). This assembly was then sintered asfollows. The assembly was first dried for 2 hours in an atmosphereconsisting of helium and 3 percent chlorine at 1000° C. followed by downdriving at 32 mm/min through a hot zone set at 1500° C. in an atmospherecomprising 50 percent nitrogen and 50 percent helium, thenre-down-driven through the hot zone at 25 mm/min in the same atmosphere,then final sintered in an atmosphere comprising 50 percent nitrogen and50 percent helium at 6 mm/min, in order to sinter the soot to an“nitrogen-seeded” randomly distributed void containing first overcladperform comprising a void-free GeO₂—SiO₂ graded index core surrounded bya “nitrogen-seeded” cladding layer. The preform was placed for 24 hoursin an argon purged holding oven set at 1000° C. The preform was thenplaced on a lathe where 1086 grams of SiO₂ soot were flame depositedonto the 1 meter long cane. This assembly was then sintered as follows.The assembly was first dried for 2 hours in an atmosphere consisting ofhelium and 3 percent chlorine at 1000° C. followed by down driving at 6mm/min through a hot zone set at 1500° C. in a 100 percent heliumatmosphere, in order to sinter the soot to an optical preform comprisinga GeO₂—SiO₂ graded index core, a “nitrogen-seeded” randomly distributedvoid containing first cladding layer and a void-free silica outercladding. The preform was placed for 24 hours in an argon purged holdingoven set at 1000° C. The preform was drawn to a 10 km length of 125micron diameter fiber at 10 m/s using a draw furnace having a hot zoneof about 8 cm length and set at approximately 2000° C. The measured OFLbandwidths of this fiber were 781 and 172 MHz-km at 850 and 1300 nm,respectively. The macrobend loss at 850 nm measured on a 20 mm diametermandrel was 0.00 dB/turn, and the NA of the fiber was 0.233.

Examples 11 and 12

127 grams of SiO₂ (0.36 g/cc density) soot were flame deposited onto a 1meter long×25.9 mm diameter solid glass cane comprising a GeO₂—SiO₂graded index core (0.96% maximum refractive index relative to puresilica) with a nearly parabolic (α=2.02) shape. This assembly was thensintered as follows. The assembly was first dried for 2 hours in anatmosphere consisting of helium and 3 percent chlorine at 1125° C.followed by fluorine doping the soot preform in an atmosphere consistingof helium and 20 percent SiF₄ at 1125° C. for 4 hours then down drivingat 14 mm/min through a hot zone set at 1480° C. in a 100 percent heliumatmosphere in order to sinter the soot to an overclad preform comprisinga germania-silica graded index core and a fluorine-doped second claddinglayer. The preform was then placed on a lathe where 283 grams of SiO₂soot were flame deposited. This assembly was then sintered as follows.The assembly was first dried for 2 hours in an atmosphere consisting ofhelium and 3 percent chlorine at 1000° C. followed by down driving at 6mm/min through a hot zone set at 1500° C. in a 100 percent heliumatmosphere, in order to sinter the soot to an optical preform comprisinga GeO₂—SiO₂ graded index core, a fluorine-doped second cladding layerand a silica outer cladding. The preform was placed for 24 hours in anargon purged holding oven set at 1000° C. For Example 11, the preformwas drawn to a 10 km length of 140 micron diameter fiber at 10 m/s usinga draw furnace having a hot zone of about 8 cm length and set atapproximately 2000° C. The measured OFL bandwidths of this fiber were1722 and 749 MHz-km at 850 and 1300 nm, respectively. The macrobendlosses measured on 15 and 20 mm diameter mandrel at 850 nm were 0.70 and0.45 dB/turn, respectively and the NA of the fiber was 0.202. ForExample 12, the preform was drawn to a 7 km length of 125 microndiameter fiber at 10 m/s using a draw furnace having a hot zone of about8 cm length and set at approximately 2000° C. The measured OFLbandwidths of this fiber were 3358 and 1059 MHz-km at 850 and 1300 nm,respectively. The macrobend losses at 850 nm measured on 15 and 20 mmdiameter mandrels were 0.70 and 0.40 dB/turn, respectively and the NA ofthe fiber was 0.202.

Example 13 Comparative

645 grams of SiO₂ (0.36 g/cc density) soot were flame deposited onto a 1meter long×26.0 mm diameter solid glass cane of GeO₂—SiO₂ graded indexcore (2.1% maximum refractive index relative to pure silica with anearly parabolic (α=2.1) shape). This assembly was then sintered asfollows. The assembly was first dried for 2 hours in an atmosphereconsisting of helium and 3 percent chlorine at 1000° C. followed by downdriving at 6 mm/min through a hot zone set at 1500° C. in a 100 percenthelium atmosphere, in order to sinter the soot to an optical preformcomprising a GeO₂—SiO₂ graded index core and a silica outer cladding.The preform was placed for 24 hours in an argon purged holding oven setat 1000° C. The preform was drawn to a 10 km length of 125 microndiameter fiber at 10 m/s using a draw furnace having a hot zone of about8 cm length and set at approximately 2000° C. The measured OFLbandwidths of this fiber were 720 and 700 MHz-km at 850 and 1300 nm,respectively. The macrobend losses at 850 nm measured on 15 and 20 mmdiameter mandrels were 0.45 and 0.35 dB/turn, respectively and the NA ofthe fiber was 0.275.

Example 14

234 grams of SiO₂ (0.36 g/cc density) soot were flame deposited onto a 1meter long×25.9 mm diameter solid glass cane comprising a GeO₂—SiO₂graded index core (2.1% maximum refractive index relative to pure silicawith a nearly parabolic (α=2.1) shape). This assembly was then sinteredas follows. The assembly was first dried for 2 hours in an atmosphereconsisting of helium and 3 percent chlorine at 1125° C. followed byfluorine doping the soot preform in an atmosphere consisting of heliumand 20 percent SiF₄ at 1125° C. for 4 hours then down driving at 14mm/min through a hot zone set at 1480° C. in a 100 percent heliumatmosphere in order to sinter the soot to an overclad preform comprisinga germania-silica graded index core and a fluorine-doped second claddinglayer. The preform was drawn into a cane with a diameter of 22 mm thatwas then placed on a lathe where 245 grams of SiO₂ soot were flamedeposited. This assembly was then sintered as follows. The assembly wasfirst dried for 2 hours in an atmosphere consisting of helium and 3percent chlorine at 1000° C. followed by down driving at 6 mm/minthrough a hot zone set at 1500° C. in a 100 percent helium atmosphere,in order to sinter the soot to an optical preform comprising a GeO₂—SiO²graded index core, a fluorine-doped second cladding layer and a silicaouter cladding. The preform was placed for 24 hours in an argon purgedholding oven set at 1000° C. The preform was drawn to a 10 km length of125 micron diameter fiber at 10 m/s using a draw furnace having a hotzone of about 8 cm length and set at approximately 2000° C. The measuredOFL bandwidths of this fiber were 588 and 253 MHz-km at 850 and 1300 nm,respectively. The macrobend losses at 850 nm measured on 15 and 20 mmdiameter mandrels were 0.17 and 0.10 dB/turn, respectively and the NA ofthe fiber was 0.291.

Table 2 illustrates the measured optical properties of the fiberdescribed in Examples 9-15. These examples illustrate that highbandwidths and low bend losses can be achieved when the annular portion30 comprises a volume V₂ with an absolute magnitude greater than about120%-μm², more preferably greater than about 160%-μm², and in some casesgreater than about 200%-μm².

As shown in Table 2, Examples 9-15 exhibit refractive index profilessimilar to those illustrated by FIG. 3. In particular, provided beloware delta 1 of core region 20, outer radius R1 of core region 20, alphaof core region 20, Delta 3 of depressed index cladding region 50, outerradius R3 of depressed index cladding region 50, profile volume V3 ofdepressed index cladding region 50. Clad radius is the outermost glassradius of the fiber as well as the outer radius of the outer annularglass cladding portion 60. Also provided is measured numerical apertureof the fiber, measured overfilled bandwidth of the fiber (measured at850 and 1300 nm), and measured 1×10 mm, 1×15 mm and 1×20 mm macrobendperformance (attenuation increase for one turn around a 10 mm, 15 mm or20 mm mandrel). Encircled flux launch (EFL) macrobend performance isprovided for example 15, and was obtained by launching an overfilledpulse into an input end of a 2 m length of InfiniCor® 50 μm opticalfiber which was deployed with a 1×25 mm diameter mandrel near themidpoint. The output end of the InfiniCor® 50 μm optical fiber wasspliced to the fiber under test, and the measured bend loss is the ratioof the attenuation under the prescribed bend condition to theattenuation without the bend. The various EFL tests involved one 180°bend or two 90° bends around a 3 mm, 4 mm, or 6 mm diameter mandrel. Insome cases the launch was directly into the center of the fiber (0 μmoffset), and in some cases the launch was offset by 10 μm from thecenter of the fiber.

TABLE 2 9 10 11 12 13 14 15 Delta1 (%) 0.96 1.02 0.96 0.96 2.1 2.1 2.0R1 (μm) 48.0 48.4 59.6 53.2 50.0 50.0 40.0 Alpha 2.11 2.16 2.03 2.03 2.12.1 2.1 Delta3 (%) 0.0 <−0.8 −0.41 −0.41 0.0 −0.4 −0.45 R3 (μm) N/A 58.463.8 57.0 N/A 55.0 45.0 V3 (%-sq. μm) 0.0 <−200 −163.4 −130.3 0.0 −204.8−191.3 Clad Radius (μm) 62.5 62.5 70 62.5 62.5 62.5 50.0 NA 0.193 0.2330.202 0.202 0.275 0.291 0.295 NA*CD (μm) 18.5 22.6 24.1 21.5 27.5 29.123.6 OFL 850 BW (MHz-km) 2315 781 1722 3358 720 586 1274 OFL 1300 BW(MHz-km) 666 172 749 1059 700 252 380 1 × 10 mm at 850 nm (dB) 1.26 0.720.37 1 × 15 mm at 850 nm (dB) N/A 0.00 0.70 0.7 0.45 0.17 1 × 20 mm at850 nm (dB) 1.04 N/A 0.45 0.4 0.35 0.1 1 × 180° at 3 mm at 850 nm, 0 μmoffset (dB) 0.24 2 × 90° at 4 mm at 850 nm, 0 μm offset (dB) 0.05 2 ×90° at 6 mm at 850 nm, 0 μm offset (dB) 0.03 1 × 180° at 3 mm at 850 nm,10 μm offset (dB) 0.28 2 × 90° at 4 mm at 850 nm, 10 μm offset (dB) 0.152 × 90° at 6 mm at 850 nm, 10 μm offset (dB) 0.01

Thus the examples disclosed herein were capable of achieving, for amultimode fiber having a core radius greater than 30 and in some casesgreater than 40 or even 50 μm, an overfilled (OFL) bandwidth of greaterthan 700 MHz-km at 850 nm, more preferably greater than 1000 MHz-km at850 nm, more preferably greater than 1500 MHz-km at 850 nm at awavelength of 850 nm. These high bandwidths can be achieved while stillmaintaining a 1 turn 10 mm diameter mandrel wrap attenuation increase ata wavelength of 850 nm, of less than 1.5 dB, more preferably less than1.0 dB, even more preferably less than 0.8 dB, and most preferably lessthan 0.5 dB. These high bandwidths can also be achieved while alsomaintaining a 1 turn 20 mm diameter mandrel wrap attenuation increase ata wavelength of 850 nm, of less than 1.0 dB, more preferably less than0.7 dB, and most preferably less than 0.5 dB. Such fibers are furthercapable of simultaneously providing a numerical aperture (NA) greaterthan 0.2 (more preferably greater than 0.24), an OFL bandwidth at 850 nmgreater than 1200 MHz-km (more preferably greater than 1500), and a 1turn 15 mm diameter mandrel wrap attenuation increase at a wavelength of850 nm, of less than 1.2 dB (more preferably less than 1.0 dB).

Example 15 illustrates a design capable of achieving, for a multimodefiber having a core radius greater than 30 and in some cases greaterthan 35 or even 40 μm, an overfilled (OFL) bandwidth of greater than 500MHz-km at 850 nm, more preferably greater than 700 MHz-km at 850 nm,more preferably greater than 1000 MHz-km at 850 nm at a wavelength of850 nm. These high bandwidths can be achieved while still maintaining a1×180° turn 3 mm diameter mandrel wrap attenuation increase at awavelength of 850 nm, of less than 1.0 dB, more preferably less than 0.5dB, even more preferably less than 0.3 dB. These high bandwidths canalso be achieved while still maintaining a 2×90° turn, 4 mm diametermandrel, 0 μm offset wrap attenuation increase at a wavelength of 850nm, of less than 0.5 dB, more preferably less than 0.2 dB, even morepreferably less than 0.1 dB. These bend losses and bandwidths areachieved when the input signal is either aligned with the center of thefiber, or when the input signal is launched with an offset of 5 or even10 μm relative to the center of the fiber. Such fibers are furthercapable of simultaneously providing a numerical aperture (NA) greaterthan 0.24 (preferably greater than 0.28), an OFL bandwidth at 850 nmgreater than 500 MHz-km (more preferably greater than 1500), and a1×180° turn 3 mm diameter mandrel wrap, 0 μm offset attenuation increaseat a wavelength of 850 nm, of less than 0.5 dB (preferably less than 0.3dB).

Example 16

A thin layer of hermetic coating with a thickness of approximately 5 nmwas applied to the surface of a glass fiber made in accordance withExample 15. The carbon coating was applied using a reactor similar tothat described in U.S. Pat. No. 5,346,520, which is incorporated hereinin its entirety by reference. The coating was formed by pyrolyticdeposition of carbon onto the waveguide fiber as the fiber emerged fromthe hot zone of the draw furnace. The fiber passed from the hot zoneinto a controlled environment chamber where a carbon containing compoundreacted to produce a carbon layer on the waveguide surface. A polymerouter coating was then applied onto the surface of the hermetic coating.

The hermetically coated waveguide was tested to determine the Weibullstrength distribution and dynamic fatigue constants as a function of theresistivity of the carbon layer, yielding the failure probability datagiven in Table 3. The average Weibull slope is 87, which indicates thatthe hermetically coated fiber will have a tight distribution of breakingstrengths. The stress value corresponding to a Weibull failureprobability of 15% is greater than 440 kpsi for all samples tested andgreater than 480 kpsi in preferred embodiments. The dynamic fatiguefactor, n, is greater than 50, preferably greater than 100 and morepreferably greater than 150. The resistance of the hermetic carbon layeris preferably between 90 and 170 kohm/cm, more preferably between 110and 160 kohm/cm, most preferably between 120 and 150 kohm/cm.

TABLE 3 Resistance 15% Failure n (kohm/cm) Weibull Stress Value  90 4341101 100 469 778 120 497 190 130 491 192 140 492 211.9 150 499 118 160511 118.9 170 501 80.4 180 492 58.9 190 491 52.8 200 495 39.2

The fibers disclosed herein may have a variety of protective coatings210 applied onto the outer glass cladding. For example, the protectivecoating 210 may include a first relatively hard protective coatingthereon. For example, the coating may be a cured polymeric layer whichwhen cured exhibits a Shore D hardness of greater than 50, 55, 60, or65. Examples of such materials may be found, for example, in U.S. Pat.No. 4,973,129, the entire specification of which is hereby incorporatedby reference. Alternatively, the relatively hard coating may be appliedas an hermetic carbon coating. The first relatively hard coating layermay be applied at a thickness which is sufficient to result in an outerthickness of between about 120 and 130 microns, more preferably betweenabout 124 and 126 microns.

If desired, conventional primary and secondary coatings may optionallybe applied over the first hard protective coating. The primary coatingserves as a buffer to cushion and protect the glass fiber core when thefiber is bent, cabled, or spooled; but it also protects the glasssurface from water adsorption, which can promote crack growth andincrease static fatigue that result in failure. The secondary coating isapplied over the primary coating and functions as a tough, protectiveouter layer that prevents damage to the glass fiber during processingand use. For example, the primary coating may have a Young's modulus ofabout 0.1 to about 3 MPa and/or a T_(g) of about −100° C. to about −25°C. As used herein, the Young's modulus of a cured primary or secondintermediate coating material is measured using a pull-out type in situmodulus test as described in Steeman et al., “Mechanical Analysis of thein-situ Primary Coating Modulus Test for Optical Fibers,” in Proc. ofthe 52^(nd) International Wire and Cable Symposium (IWCS, Philadelphia,USA, Nov. 10-13, 2003), Paper 41. A number of suitable primary coatingsare disclosed, for example, in U.S. Pat. Nos. 6,326,416 to Chien et al.,6,531,522 to Winningham et al., 6,539,152 to Fewkes et al., 6,563,996 toWinningham, 6,869,981 to Fewkes et al., 7,010,206 and 7,221,842 to Bakeret al., and 7,423,105 to Winningham, each of which is incorporatedherein by reference in its entirety. In some embodiments, conventionalprimary and secondary coatings are applied over the first relativelyhard protective layer described above. In such embodiments, the firstrelatively hard protective layer preferably exhibits a strong adhesionthat enables the primary and secondary coatings to be easily strippedtherefrom, leaving the 125 micron diameter first protective layer forinsertion into a connector or other component capable of receiving a 125micron optical fiber.

Suitable primary coating compositions include, without limitation, about25 to 75 weight percent of one or more urethane acrylate oligomers;about 25 to about 65 weight percent of one or more monofunctionalethylenically unsaturated monomers; about 0 to about 10 weight percentof one or more multifunctional ethylenically unsaturated monomers; about1 to about 5 weight percent of one or more photoinitiators; about 0.5 toabout 1.5 pph of one or more antioxidants; about 0.5 to about 1.5 pph ofone or more adhesion promoters; and about 0.01 to about 0.5 pph of oneor more stabilizers.

Other suitable primary coating compositions include about 52 weightpercent polyether urethane acrylate (BR 3741 from Bomar SpecialtiesCompany), between about 40 to about 45 weight percent of polyfunctionalacrylate monomer (Photomer 4003 or Photomer 4960 from Cognis), between 0to about 5 weight percent of a monofunctional acrylate monomer(caprolactone acrylate or N-vinylcaprolactam), up to about 1.5 weightpercent of a photoinitiator (Irgacure 819 or Irgacure 184 from CibaSpecialty Chemical, LUCIRIN® TPO from BASF, or combination thereof), towhich is added about 1 pph adhesion promoter(3-acryloxypropyltrimethoxysilane), about 1 pph antioxidant (Irganox1035 from Ciba Specialty Chemical), optionally up to about 0.05 pph ofan optical brightener (Uvitex OB from Ciba Specialty Chemical), andoptionally up to about 0.03 pph stabilizer (pentaerythritoltetrakis(3-mercaptoproprionate) available from Sigma-Aldrich).

Exemplary primary coating compositions include, without limitation, thefollowing formulations:

-   (1) 52 weight percent polyether urethane acrylate oligomer (BR 3741,    Bomar Specialty), 40 weight percent ethoxylated (4) nonylphenol    acrylate (Photomer 4003, Cognis Corp.), 5 weight percent N-vinyl    pyrollidinone, 1.5 weight percent bis(2,4,6-trimethyl    benzoyl)phenyl-phosphine oxide (Irgacure 819, Ciba Specialty), 1.5    weight percent 1-hydroxycyclohexylphenyl ketone (Irgacure 184, Ciba    Specialty), 1 pph thiodiethylene    bis(3,5-di-tert-butyl)-4-hydroxyhydrocinnamate (Irganox 1035, Ciba    Specialty), and 1 pph 3-acryloxypropyltrimethoxysilane;-   (2) 52 weight percent polyether urethane acrylate oligomer (BR 3741,    Bomar Specialty), 40 weight percent ethoxylated (4) nonylphenol    acrylate (Photomer 4003, Cognis Corp.), 5 weight percent N-vinyl    caprolactam, 1.5 weight percent bis(2,4,6-trimethyl    benzoyl)phenyl-phosphine oxide (Irgacure 819, Ciba Specialty), 1.5    weight percent 1-hydroxycyclohexylphenyl ketone (Irgacure 184, Ciba    Specialty), 1 pph thiodiethylene    bis(3,5-di-tert-butyl)-4-hydroxyhydrocinnamate (Irganox 1035, Ciba    Specialty), and 1 pph 3-acryloxypropyltrimethoxysilane;-   (3) 52 weight percent polyether urethane acrylate oligomer (BR3731,    Sartomer Co.), 45 weight percent ethoxylated (4) nonylphenol    acrylate (SR504, Sartomer Co.), 3 weight percent    (2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide    (Irgacure 1850, Ciba Specialty), 1 pph thiodiethylene    bis(3,5-di-tert-butyl)-4-hydroxyhydrocinnamate (Irganox 1035, Ciba    Specialty), 1 pph bis(trimethoxysilylethyl)benzene adhesion    promoter, and 0.5 pph polyalkoxypolysiloxane carrier (Tegorad 2200,    Goldschmidt); and-   (4) 52 weight percent polyether urethane acrylate oligomers (BR3731,    Sartomer Co.), 45 weight percent ethoxylated (4) nonylphenol    acrylate (Photomer 4003, Cognis Corp.), 3 weight percent    (2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide    (Irgacure 1850, Ciba Specialty), 1 pph thiodiethylene    bis(3,5-di-tert-butyl)-4-hydroxyhydrocinnamate (Irganox 1035, Ciba    Specialty), 1 pph bis(trimethoxysilylethyl)benzene adhesion    promoter, and 0.5 pph tackifier (Unitac R-40, Union Camp);-   (5) 52 weight percent polyether urethane acrylate oligomers (BR3731,    Sartomer Co.), 45 weight percent ethoxylatednonylphenol acrylate    (SR504, Sartomer Co.), and 3 weight percent    (2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide    (Irgacure 1850, Ciba Specialty); and-   (6) 52 weight percent urethane acrylate oligomer (BR3741, Bomar),    41.5 weight percent ethoxylated nonyl phenol acrylate monomer    (Photomer 4003, Cognis), 5 weight percent caprolactone acrylate    monomer (Tone M-100, Dow), 1.5 weight percent Irgacure 819    photoinitiator (Ciba), 1 pph thiodiethylene    bis(3,5-di-tert-butyl)-4-hydroxyhydrocinnamate (Irganox 1035, Ciba    Specialty), 1 pph 3-acryloxypropyltrimethoxysilane (Gelest), and    0.032 pph pentaerythritol tetrakis (3-mercaptopropionate) (Aldrich).

The secondary coating material is typically the polymerization productof a coating composition that contains urethane acrylate liquids whosemolecules become highly cross-linked when polymerized. Secondary coatinghas a high Young's modulus (e.g., greater than about 0.08 GPa at 25° C.)and a high T_(g) (e.g., greater than about 50° C.). The Young's modulusis preferably between about 0.1 GPa and about 8 GPa, more preferablybetween about 0.5 GPa and about 5 GPa, and most preferably between about0.5 GPa and about 3 GPa. The T_(g) is preferably between about 50° C.and about 120° C., more preferably between about 50° C. and about 100°C. The secondary coating has a thickness that is less than about 40 μm,more preferably between about 20 to about 40 μm, most preferably betweenabout 20 to about 30 μm.

Other suitable materials for use as secondary coating materials, as wellas considerations related to selection of these materials, are wellknown in the art and are described in U.S. Pat. Nos. 4,962,992 and5,104,433 to Chapin, each of which is hereby incorporated by referencein its entirety. As an alternative to these, high modulus coatings havealso been obtained using low oligomer content and low urethane contentcoating systems, as described in U.S. Pat. Nos. 6,775,451 to Botelho etal., and 6,689,463 to Chou et al., each of which is hereby incorporatedby reference in its entirety. In addition, non-reactive oligomercomponents have been used to achieve high modulus coatings, as describedin U.S. Application Publ No. 20070100039 to Schissel et al., which ishereby incorporated by reference in its entirety. Outer coatings aretypically applied to the previously coated fiber (either with or withoutprior curing) and subsequently cured, as will be described in moredetail hereinbelow. Various additives that enhance one or moreproperties of the coating can also be present, including antioxidants,catalysts, lubricants, low molecular weight non-crosslinking resins,stabilizers, surfactants, surface agents, slip additives, waxes,micronized-polytetrafluoroethylene, etc. The secondary coating may alsoinclude an ink, as is well known in the art.

Suitable outer coating compositions include, without limitation, about 0to 20 weight percent of one or more urethane acrylate oligomers; about75 to about 95 weight percent of one or more monofunctionalethylenically unsaturated monomers; about 0 to about 10 weight percentof one or more multifunctional ethylenically unsaturated monomers; about1 to about 5 weight percent of one or more photoinitiators; and about0.5 to about 1.5 pph of one or more antioxidants.

Other suitable outer coating compositions include, without limitation,about 10 weight percent of a polyether urethane acrylate oligomer (KWS4131 from Bomar Specialty Co.), about 72 to about 82 weight percentethoxylated (4) bisphenol A diacrylate monomer (Photomer 4028 fromCognis), about 5 weight percent bisphenol A diglycidyl diacrylate(Photomer 3016 from Cognis), optionally up to about 10 weight percent ofa diacrylate monomer (Photomer 4002 from Cognis) or N-vinylcaprolactam,up to about 3 weight percent of a photoinitiator (Irgacure 184 from CibaSpecialty Chemical, or Lucirin® TPO from BASF, or combination thereof),to which is added about 0.5 pph antioxidant (Irganox 1035 from CibaSpecialty Chemical).

Exemplary outer coating compositions include, without limitation, thefollowing formulations:

-   (1) 40 weight percent urethane acrylate oligomer (CN981, Sartomer    Company, Inc.), 17 weight percent propoxylated (3) glyceryl    triacrylate monomer (SR9020, Sartomer Inc.), 25 weight percent    pentaerythritol tetraacrylate (SR295, Sartomer Inc.), 15 weight    percent ethoxylated (2) bisphenol A diacrylate monomer (SR349,    Sartomer Inc.), and 3 weight percent of 1-hydroxycyclohexyl phenyl    ketone and bis(2,6-dimethoxybenrzoyl)-2,4,4-trimethylpentyl    phosphine oxide blend (Irgacure 1850, Ciba Specialty Chemical); and-   (2) 10 weight percent polyether urethane acrylate (KWS 4131, Bomar),    5 weight percent bisphenol A diglycidyl diacrylate (Photomer 3016,    Cognis), 82 weight percent ethoxylated (4) bisphenol A diacrylate    (Photomer 4028, Cognis), 1.5 weight percent Lucirin TPO    photoinitiator (BASF), 1.5 weight percent 1-hydroxycyclohexylphenyl    ketone (Irgacure 184, Ciba), and 0.5 pph thiodiethylene    bis(3,5-di-tert-butyl-4-hydroxy)hydrocinnamate antioxidant (Irganox    1035, Ciba Specialty Chemical).

The first intermediate coating 15 is typically the polymerizationproduct of a coating composition that affords a relatively higherYoung's modulus and a relatively higher T_(g) as compared to the Young'smodulus and T_(g) of the primary coating. The Young's modulus ispreferably between about 0.1 GPa and about 2 GPa, more preferablybetween about 0.2 GPa and about 1 GPa, and most preferably between about0.3 GPa and about 1 GPa. The T_(g) is preferably between about 0° C. andabout 60° C., more preferably between about 10° C. and about 60° C.,most preferably between about 10° C. and about 50° C. The firstintermediate coating has a thickness that is less than about 25 μm, morepreferably less than about 20 μm, even more preferably less than about15 μm, and most preferably in the range of about 5 μm to about 10 μm.

It is to be understood that the foregoing description is exemplary ofthe invention only and is intended to provide an overview for theunderstanding of the nature and character of the invention as it isdefined by the claims. The accompanying drawings are included to providea further understanding of the invention and are incorporated andconstitute part of this specification. The drawings illustrate variousfeatures and embodiments of the invention which, together with theirdescription, serve to explain the principals and operation of theinvention. It will become apparent to those skilled in the art thatvarious modifications to the preferred embodiment of the invention asdescribed herein can be made without departing from the spirit or scopeof the invention as defined by the appended claims.

1. A multimode optical fiber comprising: a graded index core comprisinga core refractive index delta percent Δ₁, said core comprising a coreradius greater than 30 microns; and a depressed index cladding regionsurrounding said core and comprising refractive index delta percent Δ₃,wherein Δ₃ is less than about −0.1% and said depressed region has awidth of at least 1 micron, wherein Δ₁>Δ₃, and said fiber comprises atotal outer diameter of less than 120 microns.
 2. The multimode fiber ofclaim 1, wherein said fiber is comprised of glass, and furthercomprising an outer cladding region comprising refractive index delta Δ₄said outer cladding region surrounding said depressed index claddingregion.
 3. The multimode fiber of claim 1, wherein said depressed indexcladding region is directly adjacent the core and has a refractive indexdelta less than about −0.2% and a width of at least 2 microns.
 4. Themultimode fiber of claim 1, further comprising an inner cladding regioncomprising Δ₂, wherein said inner cladding region surrounds said coreand said depressed cladding region surrounds said inner cladding region,wherein Δ₁>Δ₂>Δ₃, and said inner cladding region is less than 4 micronswide.
 5. The multimode fiber of claim 1, wherein said core comprises acore radius greater than 35 microns.
 6. The multimode fiber of claim 5,wherein said fiber is comprised of glass, said core comprises a glasscore radius less than 45 microns, and said total outer glass diameter isless than 110 microns.
 7. The multimode fiber of claim 1, wherein saidfiber exhibits an overfilled bandwidth at 850 nm which is greater than500 MHz-km.
 8. The fiber of claim 1, wherein said fiber further exhibitsa 1 turn 15 mm diameter mandrel wrap attenuation increase, of less thanor equal to 0.5 dB/turn at 850 nm.
 9. The fiber of claim 1, wherein saidfiber further exhibits a 1 turn 15 mm diameter mandrel wrap attenuationincrease, of less than or equal to 0.25 dB/turn at 850 nm.
 10. The fiberof claim 1, wherein said fiber further exhibits a 1 turn 15 mm diametermandrel wrap attenuation increase, of less than or equal to 0.1 dB/turnat 850 nm.
 11. The fiber of claim 1, wherein said depressed-indexannular portion has a width less than 10 microns.
 12. The multimodefiber of claim 1, wherein said fiber exhibits an overfilled bandwidth at1300 nm which is greater than 200 MHz-km.
 13. The multimode fiber ofclaim 1, wherein said fiber exhibits a numerical aperture greater than0.2.
 14. The multimode fiber of claim 1, wherein said fiber exhibits anumerical aperture greater than 0.24.
 15. The multimode fiber of claim1, wherein said fiber exhibits an overfilled bandwidth at 1300 nm whichis greater than 500 MHz-km.
 16. The multimode fiber of claim 1, whereindelta 2 is less than −0.3%.
 17. The fiber of claim 2 wherein the maximumrefractive index delta of the graded index glass core is greater than0.8% and less than 2.2%.
 18. A multimode optical fiber comprising: agraded index core having a radius greater than 35 microns; and a firstinner cladding comprising a depressed-index annular portion, saiddepressed-index annular portion having a refractive index delta lessthan about −0.2% and a width of at least 1 micron, and said fiberfurther exhibits a 1 turn 15 mm diameter mandrel wrap attenuationincrease, of less than or equal to 0.25 dB/turn at 850 nm.
 19. Themultimode fiber of claim 18 further comprising a numerical aperture ofgreater than 0.185.
 20. The multimode fiber of claim 18, wherein saidfiber is comprised of glass, and wherein the outer glass diameter ofsaid fiber is less than 120 microns.
 21. The multimode fiber of claim18, wherein said fiber further exhibits an overfilled bandwidth greaterthan 500 MHz-km at 850 nm.
 22. The multimode fiber of claim 17 furthercomprising a numerical aperture of greater than 0.24.
 23. The multimodefiber of claim 17 further comprising a numerical aperture of greaterthan 0.28.
 24. The multimode fiber of claim 17 further comprising a1×180° turn 3 mm diameter mandrel wrap attenuation increase at awavelength of 850 nm, of less than 0.5 dB for an encircled flux launch.25. The multimode fiber of claim 17 further comprising a 2×90° turn 4 mmdiameter mandrel wrap attenuation increase at a wavelength of 850 nm, ofless than 0.2 dB for an encircled flux launch.
 26. The multimode fiberof claim 1, further comprising a coating of carbon on the outer surfaceof said fiber, said coating having a thickness no greater than about100° A, and wherein the multimode fiber has a dynamic fatigue constantgreater than 50, and at least one polymer coating surrounding and incontact with said carbon coating.
 27. The multimode fiber of claim 25,wherein said fiber exhibits a 15% Weibull failure probability greaterthan 400 kpsi.
 28. The multimode fiber of claim 25, further comprising adynamic fatigue constant greater than
 100. 29. The multimode fiber ofclaim 20, further comprising an NA*CD greater than
 20. 30. The multimodefiber of claim 1, further comprising an NA*CD greater than
 20. 31. Themultimode fiber of claim 1, further comprising a coating thereon havinga Shore D hardness of greater than 50, said coating having an outerdiameter between 120 and 130 microns.